This invention relates to a video conversion device supplied with a compression coded signal given by subjecting an original video signal to differential coding. The video code conversion device is for converting the compression coded signal into a DCT (discrete cosine transform) coded signal identical with a coded signal given by subjecting the original video signal to DCT coding. The video code conversion device is for use in combination with a reproducing unit which reproduces the original video signal from the DCT coded signal.
When an image is digitized and recorded in a recording medium such as a CD-ROM (Compact Disk Read Only Memory) and a hard disk, the image is generally compressed into a compressed signal to be recorded so that an amount of produced data is not enormously large.
Among various compression coding systems, a DCT coding system is frequently used and is adopted in an international standard coding system such as JPEG (Joint Photographic Expert Group) and MPEG (Moving Pictures Expert Group). The DCT coding system produces a DCT coded signal given by subjecting an original video signal to DCT coding. A differential coding system, in which processing is relatively simple, is also frequently used although it is not an international standard. The differential coding system produces a compression coded signal given by subjecting the original video signal to differential coding.
Thus, there are international standards such as the JPEG and the MPEG as the DCT coding system. Accordingly, reproduction is possible by any reproducing unit which is based on the international standard. However, inverse DCT or inverse quantization processing imposes a large load upon a CPU (central processing unit). This means that high-speed reproduction can not be carried out unless the CPU is operable at a high speed. On the other hand, in the differential coding system, the load imposed upon the CPU is small because calculation of the differential values alone is required. Even with a low-speed CPU, high-speed reproduction is possible. However, there are many kinds of differential coding systems because no standard exists. This means that reproduction can not be carried out unless a specific reproducing unit specific to each system is used.
Instead of providing such a specific reproducing unit for each of various differential coding systems, it is proposed to transform the compression coded signal representative of differential codes into the DCT coded signal representative of DCT codes so that reproduction is carried out by a standard reproducing unit which reproduces the original video signal from the DCT coded signal given by the JPEG and the MPEG.
In such a device, it is necessary to expand the compression coded signal into an expanded signal (namely, the original video signal) at a high speed and then to compress the original video signal into the DCT coded signal at a high speed. If the CPU has a low processing speed and high-speed conversion is impossible, the image can not be reproduced in realtime. For example, it is assumed that the JPEG or the MPEG reproducing unit requires 20 milliseconds for reproduction of one frame. In this event, code conversion of one frame must be carried out within about 46 milliseconds in order to achieve a processing rate of 15 frames per second. If expansion of the compression coded signal into the original video signal requires 10 milliseconds and compression of the original video signal into the DCT coded signal requires 50 milliseconds, a code conversion processing time is equal to 60 milliseconds. Thus, a delay of 14 milliseconds is caused upon reproduction of the image of one frame.
In order to carry out the code conversion at a high speed, it is necessary to control an amount of the compression codes in the DCT coding system. A conventional technique is disclosed in Japanese Patent Unexamined Publications Nos. 167868/1992 and 343578/1992. In the disclosed systems, high-speed video compression is achieved by interrupting variable-length coding so as not to exceed the amount of codes assigned to each block. On the other hand, Japanese Patent Unexamined Publications Nos. 341063/1992 and 341065/1992 disclose the systems in which a plurality of quantization processings are carried out in parallel and an optimum one is coded. Thus, high-speed video compression is achieved.
In the above-mentioned video compression, it is possible to control the amount of the compression codes. However, such control is not carried out in specific concern to the processing ability of the reproducing unit. Accordingly, realtime reproduction is impossible with the reproducing unit having a low processing speed. On the other hand, a high-speed reproducing unit is allowed to reproduce no other codes than those having a predetermined picture quality although it has an ability of reproducing high-quality codes.